1. Field of the Invention
The present invention relates to a desensitizing ink for pressure sensitive copying sheets having excellent desensitizing effects and improved yellowing resistance.
2. Prior Art
Usually, pressure sensitive copying paper consists of a combination of a top sheet (CB) with a coated layer of micro capsules containing electron-donating colorless or pale colored leuco dyes dissolved in an organic solvent (capsule oil) applied to its back surface, and a bottom sheet (CF) with a coated layer containing electron accepting developers applied to its front surface. The top sheet is superposed on the bottom sheet thus the coated surfaces face each other. When pressure is applied by a ball point pen or a typewriter, the capsules are ruptured and the capsule oil containing leuco dyes are transferred to the developer layer, so that a color-developing reaction takes place and printed records are obtained. Further, a plurality of copies can be obtained if one or more middle sheets are placed between the top and bottom sheets, the middle sheet (CFB) having the developer layer applied to its front surface and the capsules layer applied to its back surface.
When the pressure sensitive sheet is to be used for account slips or some other formats, there may be portions that do not require the copying function in the layout. To eliminate the color-developing function from the portions where no copying function is required, a desensitizing ink is usually printed on those areas of the developer layer of the middle sheet or bottom sheet.
Large amounts of the desensitizing ink are currently being used in accordance with the widespread use of pressure sensitive copying sheets, and a variety of types of ink suited to various printing systems have been placed on the market.
Representative examples of desensitizing ink used at present include oily desensitizing inks for typographic printing, solvent-type desensitizing inks for flexographic printing, and specially prepared desensitizing inks which cure upon irradiation with ultraviolet rays.
Examples of the conventional desensitizing agent used in desensitizing inks include cationic quaternary ammonium salt (such as dodecyltrimethylammonium chloride) disclosed in Japanese Patent Publication No. 3921/1958, a tertiary amine having two polyethylene oxide groups (such as N,N-di (polyoxyethylene) ethylamine) disclosed in Japanese Patent Publication No. 29546/1971, a polyoxyethylene monoalkyl ester (such as polyoxyethylene oleyl ester) disclosed in Japanese Patent Publication No. 38201/1972, a polyethylene glycol alkylphenyl ether (such as polyethylene glycol nonylphenyl ether) disclosed in Japanese Patent Publication No. 8288/1974, and the like. The desensitizing ink is generally printed by typographic printing (such as rubber relief printing), flexographic printing or gravure printing systems.
The conventional desensitizing ink, however, has defects of insufficient desensitizing effect or of yellowing properties when exposed to light, and the object of the ink is not satisfactorily accomplished.
On the other hand, the desensitizing printing is usually effected in combination with the printing of ruled lines and characters. The ruled lines and characters, in many cases, are printed by the wet offset system. Therefore, there are many users who want a desensitizing ink that can be printing by a two-color offset printer. Wet offset printing consists of supplying wetting water to hydrophilic blank portions of the printing plate, and supplying ink from an inking roller to oleophilic (water-repellant) portions free from damping water, so that the ink is transferred to the paper via the blanket to effect the printing. Such a printing system, therefore, requires an ink having a strong oleophilic property. When a hydrophilic ink is used, the paper is not evenly printed since the ink is poorly supplied to the oleophilic portions, or the ink mixes with the wetting water, contaminating the wetting water device and the nonprinted portions.